The present invention relates to a heterojunction bipolar transferred electron tetrode. The heterojunction bipolar transferred electron tetrode is an improvement of a known heterojunction bipolar transferred electron triode having three terminals of the cathode, base and anode, and further has a separate fourth terminal that operates independently of the three terminals.
A heterojunction bipolar transferred electron triode is a three-terminal device in which high-frequency Gunn-Hilsum oscillations are generated (xe2x80x9cTransferred electron induced current instabilities in heterojunction bipolar transistorsxe2x80x9d, V. A. Posse, B. Jalali and A. F. J. Levi, Appl. Phys. Lett. 66 (24), Jun. 12, 1995). The device has a structure somewhat similar to a conventional npn heterojunction bipolar transistor, although the mechanism of its operation is completely different and a very specific design must be employed for the active region of the device (corresponding to the collector region of the bipolar transistor) to achieve device operation (xe2x80x9cDemonstration of a 77-GHz Heterojunction Bipolar Transferred Electron Devicexe2x80x9d, J. K. Twynam, M. Yagura, N. Takahashi, E. Suematsu and H. Sato, IEEE Electron Device Lett., 21 (1), 2000).
A heterojunction bipolar transferred electron triode can operate as a self-oscillating mixer of a high-frequency receiver. For example, in the down converter mode the RF (radio-frequency) signal is input into the cathode, the base is grounded and the IF (intermediate frequency) output signal, which is obtained as a result of mixing of the RF signal and the LO (local oscillation) signal generated by Gunn-Hilsum oscillations in the active layer, is extracted from the anode. In this free-running mode of operation the LO frequency of the device is not accurately defined and is dependent on the device structure and the external circuit impedances. This presents a problem because a small percentage error in the LO signal frequency gives a large percentage error in the down-converted IF signal frequency. For example, if the RF signal frequency is 100 GHz and the LO frequency is 102 GHz xc2x12 GHz, then the down-converted IF signal frequency will be 2 GHzxc2x12 GHz. Such a large margin of error in the IF signal frequency is not acceptable. An additional problem is that, like most oscillators, the phase noise of the LO signal is generally rather poor under free-running conditions.
The stability of the IF output signal depends directly on the stability of the LO signal. In a free-running device, however, the stability of the LO signal is usually insufficient. In order to stabilize the fundamental frequency of the LO signal at the desired value and decrease the phase noise, the technique of injection locking can be employed. In this technique a stable sub-harmonic signal is injected into the cathode, along with the RF signal. The injection locking signal power is generally much greater than the RF signal power and this can lead to the problem of unwanted transmission of the injection locking signal from the receiver antenna. This is illustrated by the circuit block diagram shown in FIG. 8. Reference numeral 801 in FIG. 8 indicates the heterojunction bipolar transferred electron triode. The transmission of the locking signal from the receiver antenna is undesirable because it can interfere with other receivers.
An object of the present invention is to provide a noble device that can solve the above problems, by improving the heterojunction bipolar transferred electron triode.
In order to accomplish the above object, the present invention provides a heterojunction bipolar transferred electron tetrode comprising:
an anode region providing a first terminal as an output terminal;
an active region in which Gunn-Hilsum oscillations are generated;
a base region providing a second terminal;
a cathode region providing a third terminal;
the anode region, active region, base region, and cathode region being provided sequentially, and
a fourth terminal which is operable independently of each of the three terminals, and wherein
the cathode region and fourth terminal are in proximity enough to each other such that one of these two terminals is usable as an input terminal and the other of these two terminals is usable as a terminal to which an electrical signal for disturbing an electric field profile or a current density in the active region is applied.
The heterojunction bipolar transferred electron tetrode of this invention has the fourth terminal which operates independently of the existing three terminals of the anode, base and cathode. This allows an injection-locking signal to be injected into a different terminal (namely, the fourth terminal or the cathode terminal) from the terminal (namely, the cathode terminal or the fourth terminal) into which the RF (or IF) signal is input, leading to good isolation between the two terminals when the heterojunction bipolar transferred electron tetrode of this invention is used as a self-oscillating mixer. Thus, the injection-locking signal is prevented from being transmitted from a receiver antenna when the receiver contains the heterojunction bipolar transferred electron tetrode down converter. Further, the electrical signal applied to the fourth terminal or the cathode terminal (third terminal) has the effect of disturbing the electric field profile or the current density in the active layer so that a sub-harmonic locking signal applied to the fourth terminal or the cathode region can be used to stabilize the Gunn oscillation (LO) signal frequency.
When the heterojunction bipolar transferred electron triode is used as a self-oscillating mixer, the LO signal is generated by Gunn-Hilsum oscillations in the active layer and the signal appears between the first terminal (anode terminal) and the second terminal (base terminal) (which is usually ground). The power of these LO oscillations is dependent on the electric field profile and the current density in the active layer, both of which are time-dependent and position-dependent. The LO signal power is dependent, therefore, on the cathode current since this affects the current density in the active layer. Since this dependence is non-linear, an RF (or IF) signal input into the cathode (third terminal) or the fourth terminal is mixed with the LO signal and the resulting IF (or RF) signal is obtained, along with the LO signal, at the output (first terminal).
The fourth terminal can take the form of a second cathode-type structure, a second base region, or a Schottky barrier gate electrode.
A minimum distance between the fourth terminal and the cathode region in a direction parallel to a plane of the interface between the base region and the active region may be less than twice a thickness of the active region in a direction perpendicular to the plane, in view of an angle at which an electron current flux spreads in the active region.
The anode region, active region, and cathode region may be made of n-type semiconductors and the base region may be made of a p-type semiconductor.
In one embodiment, the fourth terminal is constituted of a second cathode region. Each cathode region is located adjacent to the base region and has its own independent electrode. The cathode regions, base region, active region and anode region are formed of III-V semiconductor compounds containing one or more of group III elements of Ga, In or Al and one or more of group V elements of As, P or N, and the active region has a doping concentration that decreases with increasing distance from the base region. When this device is used as a self-oscillating mixer, an RF (IF) signal will be input into one of the two cathode terminals and a locking signal will be input into the other cathode terminal. This device may have additional cathode regions.
In another embodiment, the fourth terminal is constituted of a second base region. Each base region is located adjacent to the active region and has its own independent electrode. The cathode region, base regions, active region and anode region are formed of III-V semiconductor compounds containing one or more of group III elements of Ga, In and Al and one or more of group V elements of As, P and N, and the active region has a doping concentration that decreases with increasing distance from the base region. When this device is used as a self-oscillating mixer either an RF (IF) signal will be input into the cathode terminal and a locking signal input into the second base terminal, or an RF (IF) signal will be input into the second base terminal and a locking signal input into the cathode terminal. This device may have additional base regions and cathode regions which each have their own electrodes.
In still another embodiment, the fourth terminal is constituted of a Schottky barrier gate electrode formed on the active layer. The cathode region, base region, active region and anode region are formed of III-V semiconductor compounds containing one or more of group III elements of Ga, In and Al and one or more of group V elements of As, P and N, and the active region has a doping concentration that decreases with increasing distance from the base region. When this device is used as a self-oscillating mixer either an RF (IF) signal will be input into the cathode terminal and a locking signal input into the Schottky barrier gate electrode, or an RF (IF) signal will be input into the Schottky barrier gate electrode and a locking signal input into the cathode terminal. This device may have additional cathode regions, Schottky barrier gate electrodes and base regions.
Other objects, features and advantages of the present invention will be obvious from the following description.